Unable to identify USER on bash script executed by udev

Question

I have created an udeve rule (on Debian) to copy files from sdcard when it is plug in:

# only do something for sdX
KERNEL!="sd[a-z]*", GOTO="my_mount_end"
#
SUBSYSTEM=="block", ACTION=="add", ENV{PARTN}=="1",  RUN+="/usr/bin/at -M -f /usr/local/bin/SCRIPT now"    
# exit
LABEL="my_mount_end"
#

/usr/local/bin/SCRIPT is a bash script on which I want to differentiate classical use from current user and usage via the udev rule. Thus, I am trying to identify the USER that runs the script but $USER, $EUID and $(id -u) give nothing. How can I especially identify that root runs the script?